Mycorrhizae and Phosphorus Nutrition of Pine Seedlings In a Prairie Soil Nursery I By A. L.

MCCOMB2

The relation of the mycorrhizal soil fungi to the growth of conifers has long been a controversial sUbject.3 Recent experiments in nursery production, in afforestation and in extending the range of certain species of conifers (11, 14,21,32, 36, 37, 38) have reemphasized the importance of these fungi in promoting tree growth and aiding nutrition. Nonetheless, the manner in which these organisms work is still obscure, and growth stimulation of seedlings has been attributed both to effects resulting from the invasion of short roots by fungi and to the activity of the fungus in the soil adjacent to the roots. The invasion of the short roots by the fungus commonly results in the formation of ectotrophic mycorrhizae which are characterized by an external fungal mantle and an internal Hartig net (16). When the fungus is more active, intracellular haustoria (48) are formed in addition to the intercellular net, and these types of mycorrhizae have been called ectendotrophic. The nature of the fungal infection is apparently dependent to a considerable degree upon the species of fungus and the relative vigor of both the host plant and the fungus. Endotrophic mycorrhizae, which are characterized by intracellular infection, but which lack the fungal mantle and Hartig net, are apparently not common among the pines. When mycorrhizae are formed, the root system of the plant is often enlarged. According to Hatch (15) this enlargement results in better mineral nutrition. Better nutrition is derived directly from the greater number of absorbing short roots, the greater surface area of individual mycorrhizal roots, the delay in suberization of the cortex and endodermis, and the great extension of the fungal mycelium through the soil. Mycorrhizae are also claimed to enable trees to acquire an increased supply of any limiting 1 Part of a thesis entitled "Nutrition and growth of forest tree seedlings," submitted to the faculty of the Graduate College, Iowa State College. in partial fulflllment of the requirements for the degree. Doctor of Phllosophy. Project 612 of the Iowa Agricultural Experiment Station, Ames, Iowa. 2 The author is indebted to Dr. W. E. Loomis. Prof. G. B. MacDonald and Dr. W. H. Pierre of the Iowa Agricultural Experiment Station for encouragement, advice and assistance during the prosecution of this work. Dr. J. E. Sass of the Botany Department prepared some of the sections of mycorrhlzae and took some of the photomicrographs. 3 The literature on this subject has been extensively reviewed by Kelley (20). Hatch (15) and Rayner (35).

584 element (15, 43) or of water (8), and to enable the plants to absorb carbohydrates (12, 13, 48) and organic nitrogen (28). In addition, a theory of nitrogen fixation by mycorrhizal fungi has been advanced. The fact that conifers will grow well in some soils, apparently without the aid of mycorrhizae, has led to the belief that these fungi are not important for growth and are essentially weak parasites (25, 27, 40). Another view (5) is that mycorrhizal fungi, although parasitic, may indirectly benefit the host by releasing nutrients and other substances from organic matter. More recently the view has been presented that good tree growth and the formation of mycorrhizae are intimately bound up with the chain of reactions characteristic of some types of organic matter decomposition and humus formation. According to Rayner (37), some soils accumulate substances which prevent activity of mycorrhizal fungi. In these soils tree growth is very poor. Similarly Young (48) has noted that the mycorrhizae of some pines were abnormal on sandy soils low in organic matter, and that on these soils, the fused needle disease was widespread. Where mycorrhizal fungi are active, Rayner (36, 37) noted that tree roots were often stimulated before mycorrhizae were formed and she concluded that introduction of active mycorr~izal material to an unfavorable soil "leads directly to the production of growthpromoting substances in the substrate." In a later experiment, Rayner (39) found that conifer seedlings were stimulated more by addition of certain composts to the soil than by addition of nutrient salts equivalent to those found in the composts. Lindquist (23) has shown that some fungal extracts stimulated conifer seedlings while extracts from other fungi were toxic. There is some evidence that phosphorus plays an important role in the fungus-root relationship. Melin (28) showed that mycorrhizal fungi were greatly stimulated by root excretions containing phosphatides. In Australia (22) poor growth of pines was shown to be due to a phosphorus deficiency, and Young (48) presented an hypothesis regarding the role of phosphorus in relation to mycorrhizae and tree growth. According to his hypothesis mycorrhizae may furnish trees with carbohydrates. The addition of phosphorus to a soil low in this element results in increased phosphatide secretion from tree roots and increased growth of ground cover vegetation. Mycorrhizal fungi are stimulated by the excreted phosphatides and having additional vegetable detritus on which to work, are able to supply trees with significant quantities of carbohydrates. It is doubted, however, if carbohydrate nutrition is of importance on well aerated mineral soils where light is adequate for growth. The phosphorus nutrition of mycorrhizal and non-mycorrhizal pines has not been widely investigated on a quantitative basis. Mitchell (30), however, has shown that white pine in sand

585 cultures grew best at phosphorus concentrations of 300-350 ppm. Conversely, work with apple (3) and peach (9) seedlings in sand cultures showed best growth at phosphorus concentrations of 4 ppm. With various herbaceous crop plants (33, 44) maximum growth has been obtained in solution cultures where phosphorus was maintained at levels of 1.0 ppm. or below. If Mitchell's data are representative of non-mycorrhizal pines, it is evident that conifers present a special problem in phosphorus nutrition. In central Iowa some difficulties have been encountered in attempting to grow conifer seedlings in a nursery on a prairie soil. Inability to obtain satisfactory seedling growth apparently has been due to absence of. mycorrhizae and poor phosphorus nutrition. This bulletin presents a description of difficulties encountered at the nursery and some experiments carried out to ascertain the causes of poor growth. The nutrition of pine seedlings is discussed in relation to factors peculiar to this nursery. EXPERIMENTAL The Iowa State Forest Nursery is located on an O'Neil sandy loam soil which occupies a high terrace in Section 14, T. 83 N., R. 23 W., about 1 mile south and slightly east of Ames, Iowa. The surface soil varies in depth from about 8 to 13 inches, and prior to acquisition as a nursery site the tract had been used for agricultural crops. The surface soil is dark brown to greyish brown in color, has a pH of from 5.8 to 6.5 and is underlain with a sandy gravel stratum at depths ranging from about 20 to 30 inches. Analyses showed this soil to contain on a per acre4 basis 2000 pounds total nitrogen, 40 pounds dilute acid-soluble phosphorus, and 1600 pounds of exchangeable calcium. During the spring of 1937, the first crops of conifers were seeded in the nursery. The species included northern white pine, Pinus strobus L.; ponderosa pine, Pinus pOlldcrosa Doug!.; Virginia pine, Pinus virginiana Mill.; and Japanese red pine, Pilllts dcnsiflora, Sieb. and Zucc.; all of which were mulched with pine needles from a vigorous 14-year old plantation of white and red pines. Other conifers seeded but not mulched with pine needles included red or Norway pine, Pinus rcsinosa, Ait.; Austrian pine, Pinus nigra, Arnold; Douglas fir, Pscudotsltga ta.-rifolia, Brit.; and Norway spruce, Picea Abies Karst. Following germination, good stands of all species except Austrian pine were obtained, and seedling development was normal until approximately Aug. 1 when the first fascicled needles began to appear. At this time it was noted that the beds which had been mulched with pine needles were spotted in appearance, and 4 An acre of soil 6% Inches deep Is considered to contain 2 million pounds of soil materials.

seedling gro)Vth was irregular. In certain areas of these beds the seedlings continued to make vigorous growth, while in other areas they were stunted and off-color. The seedlings of species not mulched with pine needles were uniformly stunted . . As the season progressed, the difference between the good and poor spots in any bed became greater. In late September the stunted seedlings began to turn brown or reddish purple in color while the vigorous seedlings retained a normal green color. At the end of the growth period the vigorous seedlings were approximately twice as large as the stunted ones. Upon examining the seedlings from the good and poor spots in, any bed it was found that the vigorous seedlings invariably possessed an abundance of ectotrophic mycorrhizae, while stunted seedlings possessed few or none. This relationship was true of all seedlings from beds mulched with pine straw and was most pronounced with Virginia and Japanese red pines. The seedlings from beds of Austrian and red pine, Douglas fir and Norway spruce, which did not receive the needle mulch, were uniformly poor and none possessed observable ectotrophic mycorrhizae. In the winter many of the non-mycorrhizal seedlings in the mulched beds died. During the second growing season, however, the size of the spots supporting vigorous seedlings increased, and

587 many of the off-color seedlings of the previous year now regained their greenness and began to make good growth. At the end of the second season all except a few of the spots previously showing stunting contained seedlings of good vigor. Size differences were still evident but all plants were making satisfactory growth. In the beds not originally mulched with pine straw, winter mortality was very noticeable. In late spring of the second season,

588 however, a few small spots of vigorous seedlings appeared, and upon examination these seedlings were found to have mycorrhizae. These spots enlarged slowly during the growing period, but by t,he end of the second season approximately 90 percent of the Douglas fir seedlings, and only a slightly lower percentage of the Austrian pine, red pine and spruce were dead. The total loss in this instance was approximately 350,000 2-05 seedlings. The condition of the Douglas fir, the red pine and the Norway spruce seedlings is illustrated in figs. 1 and 2. INOCULATION OF NURSERY SEEDBEDS

To ascertain if the pine needles used to cover the seedbeds at the State Forest Nursery were a source of mycorrhizal fungi, inoculation of new coniferous seedbeds was attempted. In the spring of 1938 duff and humus-rich top soil were obtained from the plantation which had furnished the pine straw used to cover the 1937 seedbeds. It had been ascertained first that the trees in this plantation bore mycorrhizae. This soil was applied at the rate of 1 bushel per 250 sq. ft. of seedbed and was raked in to a depth of about 2 inches. The bed was then sown to Scots pine, Pinus sylvestris L. The author was not able to observe the beds again until the first part of September, 1938. At this time striking differences were observed between plants from inoculated and uninoculated soil. These differences are illustrated in fig. 3. At the end of the second growing season, in 1939, many of the uninoculated seedlings were dead while those that remained alive averaged less than 2.0 inches in height. At this same t;me the inoculated seedlings, where competition was not excessive in the bed, reached an average height of about 7 inches. Similarly, inoculation of seedbed soil with this same duff was attempted for other species. These species included white pine; jack pine, Pinus banksiana Lamb; and Douglas fir. The results obtained were similar to those obtained with Scots pine. In all cases a marked stimulation of growth occurred and ·mycorrhizae appeared in the inoculated but not in the uninoculated plots. NUTRIENT ABSORPTION BY INOCULATED AND UNINOCULATED PINE

Since it has been observed (14, 15, 31) that mycorrhizae may enable coniferous seedlings to increase their absorption of some plant nutrients, a study was made of the nutrient content and root development of mycorrhizal and non-mycorrhizal pines from the State Forest Nursery. This work was started in the fall of 1937 and completed in the spring of 1938. 1-0 Virginia pine stock 5 In this designation the first digit represents the number of years in the seedbed; the second the number of years in the first transplant bed.

589 was used. A block of soil containing over 200 seedlings was removed from each of a mycorrhizal and non-mycorrhizal spot in one of the seedbeds. These blocks were within 10 feet of each other in the bed, the pH of the soil varied from 6.0 to 6.2, and from all outward appearances the soils in the two blocks were similar. The soil was carefully washed. from the seedlings, and damaged plants at the edges of the blocks were discarded. From among the uninjured seedlings, a sample of 20 was chosen at random from each block for a study of the mycorrhizae and of seedling development. The remaining seedlings were used for chemical analyses. The 20 mycorrhizal and the 20 non-mycorrhizal seedlings were examined externally to determine the nature of the absorbing roots, and counts were made of the number of mycorrhizal

branches or tips and of the non-mycorrhizal short roots. The root classification of Hatch and Doak (16) was used. The same seedlings were used for the following measurements of height growth: From root collar to top of cotyledonary growth, from top of cotyledonary growth to the bud, and total height. The growth from the cotyledons to the bud was believed most useful in evaluating height growth response associated with mycorrhizae, because the initial growth effects due to the seed were probably less evident in this latter growth. Large seeds contain more food and essential nutrients (I, 30) and seedling yield has been shown to vary with seed size and weight (1, 24, 83). The seedlings used for chemical analyses were counted and green and dry weights determined. The ground tissue was then analyzed for total nitrogen, phosphorus and potassium by the methods of the Association of Official Agricultural Chemists (2). The results of all comparisons are presented in tables 1 and 2. Figures 4 and 5 illustrate the differences between mycorrhizal and non-mycorrhizal seedlings. The data are all clear cut, and there was little overlapping in the ranges displayed by individual measurements. Statistical signi ficance was determined in all cases by the methods outlined by Snedecor (41). From these data, the following points can be made:

593 1. Green and dry weights of mycorrhizal plants were double those of the non-mycorrhizal plants. , 2. Total height of mycorrhizal plants was 35 percent greater, and height growth from cotyledons to the bud 60 percent greater than that of the non-mycorrhizal seedlings. 3. The mycorrhizal plants were slightly, but significantly longer (17 percent) from root collar to cotyledons than the nonmycorrhizal plants. 4. The average mycorrhizal plant had well over 600 absorbing short roots and mycorrhizal tips or branches while the nonmycorrhizal plants had only slightly over 300. 5. The number of nonmycorrhizal short roots on mycorrhizal and non-mycorrhizal plants was about the same. 6. The extra absorbing root tips on the plants from inoculated soil were thus mycorrhizal and can be thought of as extra root tips formed as a result of the mycorrhizal stimulus. 7. Mycorrhizal plants contained (totals per plant) twice as much nitrogen and potassium and four times as much phosphorus as nonmycorrhizal plants. 8. On a percentage of dry weight basis, the seedlings with mycorrhizae containe~ twice as much phosphorus as non-mycorrhizal plants but there was little difference in the contents of nitrogen and potassium. The above data indicate that mycorrhizae, or the conditions which permit formation of mycorrhizae, were directly stimulating to root growth and activity and that plants thus stimulated were capable of absorbing increased quantities of phosphorus Fig. 5. Root systems of 1-0 Vir- an element which was ginia pine from O'Nell soil. Nonmycorrhizal (I eft) • mycorrhizal apparently limiting growth (right). in this soil.

594 RESPONSES OF JACK PINE TO NITROGEN AND PHOSPHORUS

In view of the poor color of the non-mycorrhizal seedlings started in 1937, and other indications of nutrient deficiencies, direct additions of nitrogen and phosphorus to conifer seedlings were tested. These experiments were started in the spring of 1938 before the data obtained with Virginia pine had been evaluated. Jack pine was chosen for the experiments because seed of Virginia pine was not available and because jack pine normally makes rapid growth, once past the cotyledon stage. If seedlings inherently grow slowly any fertilizer effects are partially masked by slow growth and inherent variability. The experiments started in the spring of 1938 were carried out in cypress boxes about 11 inches deep and 10 inches square, with a capacity of slightly le£s than two-thirds of a cubic foot. The soil used was O'Neil sandy loam obtained from the surface of an eroded spot in a field next to the nursery and probably represents the As or BI horizon, although the field had been cropped to corn and sorghum for the 2 years prior to collecting the soil, and horizons were mixed. This eroded soil was chosen to get material which from appearances was lower in total nitrogen than the surface soil of uneroded spots. Analyses showed it to contain approximately the same quantity (18.5 ppm.) of dilute acidsoluble phosphorus as was found in the nursery surface soil. After the soil had been thoroughly mixed it was measured out box by box and the fertilizers mixed completely with all the soil in each box. Four treatments were used: (1) Check, (2) high nitrogen-low phosphorus, (3) high nitrogen-high phosphorus, and (4) low nitrogen-high phosphorus. All boxes received a basic application of 360 pounds K 2 0 per acre as potassium sulphate. Rates of application were calculated on a volume basis. Nitrogen was added as ammonium sulphate and phosphorus as mono-calcium phosphate. The rates of application of nitrogen and phosphorus are shown in table 3. All treatments were replicated five times. Jack pine seeds were planted in each box, and the boxes were then mulched with reeds and sedges and set on planks in a small snow fence enclosure at the nursery. After emergence the seedlings were kept shaded until Aug. 1, at which time they were considered hardy enough to stand full exposure. Throughout the growing seasons they were watered to optimum. Late in November, 1938, all plants were mulched to prevent winter injury. During the spring of 1939, the mulch was removed and the boxes refertilized with nitrogen equal to that originally added, and P~?SP~orus equiva.lent to one-half the original quantity. Refertl!tzatlOn was deCIded upon" after tests made during the prevlOUS late summer indicated that most of the nitrogen either

TABLE 3.

Treatment· Check

GROWTH OF' JACK PINE IN RELATION TO SEVERAL FERT ON O'NEIL SANDY LOAM.

596 had been used or had been leached from the soil. At this time nitrogen was added as ammonium nitrate instead of the sulphate. Phosphorus was applied as a solution of mono-calcium phosphate, the half application being dissolved in one liter of water so as to obtain maximum penetration of the phosphate. Following refertilization the seedlings in each box were thinned to 12 per box to avoid serious competition among the plants. During the first growing season several of the seedlings in each of the check boxes died. About Sept. 15 of the first season it was first noted that the seedlings in one of the check boxes were definitely superior to those in the other four c he c k boxes. Fig. 6. Two-year-old jack pine seedlings This difference in~rown on O'Neil solI at different levels of nitrogen and phosphorus. Left to right; creased during the fall check. N-l and P-3. N-3 and P-3. N-3 and P-l. and was very pronounced throughout the second growing season. In the second season it was noted also that considerable variability occurred among the replications of plants receiving the low phosphorushigh nitrogen treatment. The good growth of the one check was later ascertained to be due to fortuitous inoculations with mycorrhizal fungi. During the winter of 1940 this experiment was dismantled, the seedlings examined, and heights and green and dry weights obtained. These data are presented in table 3 and are illustrated in figs. 6 and 7. Data for analysis of variance are presented in table 4. From the data the following points are evident: 1. Jack pine grew very poorly or died on this soil when unfertilized or uninoculated. 2. Total dry weight increase of the high P and N treatment over the check was about 2800 percent. 3. The major part of the growth response obtained apparently was due to phosphorus.

597 4. Conclusive evidence of the importance of nitrogen could not be derived from this experiment, but indications were that nitrogen response was obtained, if at all, only after the phosphorus requirements had been met. 5. The similarity between the growth of the inoculated check (box 101) and the low phosphorus-high nitrogen treatment suggests that, if inoculated from the first, mycorrhizal seedlings would grow as well as those of the above fertility treatment. Observation of the seedlings from uninoculated fertilized soil showed that they possessed a great abundance of light colored, apparently active absorbing root tips. These tips were definitely swollen, were sometimes branched, and superfij:ially resembled mycorrhizal roots. Study under a low power (1 OOX) microscope, however, failed to disclose any fungal man tIe, and it was tentatively concluded that these roots were either non-mycorrhizal or else were endotrophic mycorrhizae. vVhen some of these roots were later sectioned . and studied under higher magnifications they were found to be mycorrhizae of the common type, but differed from those found on the inoculated plants in that the fungal infection was strictly intercellular, the hyphae much smaller in diameter, and the man tIe more compact. To compare the phosphorus response of pine seedlings with another type of plant, oats were grown on O'Neil soil treated with various levels of nitrogen and phosphorus. On this soil where jack pine showed a 2800 percent response, 1110st of which was due to phosphorus, oat p I ant s showed a 700 percent response to nitrogen and an 80 percent response to phosphorus. These results suggest that there is a vast difference, either in the feeding power or in the requirements of pines and oats as far as phosphorus and nitrogen are concerned, and that without mycorrFig. 7. Two-year-old jack pine seedlings from O'Neil soiL Nonhizae pines acquire phosmycorrhizal (lett) and mycorrhizal phorus with difficulty. (right).

Simultaneously with the experiment with nitrogen and phosphorus, jack pine was grown on several Iowa surface soils. The soils used were Webster silty clay loam obtained from the Agronomy farm of the Iowa State College, D-Clarion6 loam, and Ames silt loam. The Webster is a prairie soil, the Ames a forest soil obtained near the Ledges State Park, and the DClarion soil used was a degraded prairie soil obtained from the Agronomy farm. The setup for this experiment was similar to that for jack pine on O'Neil soil. All soils were left untreated. Data concerning these soils are presented in table 5. The results of this experiment are presented in table' 6 and illustrated in fig. 8. On the Webster soil which had a pH of 7.8 and an abundance of free calcium carbonate, the seeds germinated and the seedlings lived in a chlorotic stunted state for from 1 to 4 months and then died. N one was alive in the late fall. On the Clarion and Ames soils, both of which were slightly acid and originally supported a mixed stand of oaks, hickory and elm, seedling growth was very good and in fact superior to the growth obtained on the fertilized O'Neil soil. All plants grown on D-Clarion and Ames soils possessed mycorrhizae and were vigorous and of a healthy ·color. EFFECT OF SOIL INOCULATION AND FERTILIZATION ON THE ROOT SYSTEMS OF PINE

Microscopic studies were made of the root systems of pines to compare the effect of inoculation .with that of fertilization. This work was done with jack pine grown in 1938-39 and white 6 D-Clarlon refers to Clarion soli, originally prairie, which l1ad been partially leached and degraded as a result of occupation by forest.

599 TABLE 5.

SOME CHEMICAL CHARACTERISTICS OF THE SOILS USED TO GROW PINUS BANKSIANA.

pine grown during 1940-41. The fertilized plants stuclied received phosphorus, nitrogen and potassium. A wide variety in the type of mycorrhizal roots and in the degree of infection was revealed by this study. Root tips from seedlings of jack and white pine from inoculated unfertilized O'Neil soil were (1) simple uninfected short roots, (2) ectoTABLE 6.

Soil

Webster

D·CJarion

Ames

O'Neil' NPK

* The

GROWTH OF JACK PINE ON SEVERAL UNTREATED SURFACE SOILS AND ON FERTILIZED O'NEIL SUBSOIL. Box no.

600 trophic mycorrhizae, or (3) ectendotrophic mycorrhizae (those possessing both inter- and intra-cellular infection). On white pine two types of fungal mantles were observed, one of which was relatively t h i nand light brown in color while the other was heavy and dull grey. In both cases branching was dichotomous. Among the mycorrhizae there was no sharp dividing line bet wee n ecto- and ectendotrophic types, and apparently the same fun gus was found inside as well as between the cells. I f this is the case, then differences in mycorrhizal type in inoculated soil represented di fferences in degree of infection and perhaps in relative vigor of host and fungus. In instances where intracellular infection was severe, Fig. 8. Two-year-old jack pine seedlings on several soils. Left to right : D-Clarion, the walls of cortical O'Neil + NPK, Ames. B elow: O'Neil, check. cells were often irregular and appeared partially broken clown. Sometimes one side of a root possessed intercellular hyphae while the other side was uninfected. The non-mycorrhizal short roots possessed small, slightly swollen, light brown absorbing tips while both of the mycorrhizal types exhibited considerable hypertrophy, and the light colored tips were four or more times longer than those of the uninfected roots. As previously indicated, application of phosphorus to uninoculated O'Neil soil resulted in good growth of seedling pines and in the formation of an extensive absorbing system with enlarged short roots. Microscopic sectioning of these enlarged roots showed them to be ectotrophic mycorrhi'zae. They possessed both a well developed mantle and the Hartig net, but none had intracellular infection. These mycorrhizae differed from those on inoculated plants in that the intercellular net often appeared to be composed of two or more mycelial strands each of which was only about half as large in diameter as those from inoculated plants. The strands of mycelium making up the mantle were

602 also small in diameter, and the mantle was much more compact than that found on the inoculated seedlings. Because of the compactness of the mantle and the small diameter of the mycelial threads, the mycorrhizal character of these roots was not detected with external examination using a low power microscope (lOmq, and at first they were thought to be either endotrophic mycorrhiiae or uninfected roots. The fertilized uninoculated plants had about the same proportion of uninfected roots as was found on inoculated unfertilized plants. In view of these results it seems probable that the O'Neil soil originally contained fungi capable of forming- mycorrhizae, but that soil or plant conditions favorable to the formation of mycorrhizae were not obtained prior to fertilization with phosphorus. The mycorrhizal fungi from these roots have not been cultured and their identities are unknown, but in view of the facts presented and the differences in thickness of the mycelium, it seems reasonable to assume that at least two species were involved, one of which may have existed in a subdominant state in the unfertilized soil. Several non-mycorrhizal short roots from plants neither fertilized nor inoculated were sectioned for microscopic examination. None of these roots showed intra- or inter-cellular-infection and therefore cannot be considered pseudo-mycorrhizal. All of such roots were heavily impregnated with materials not found present in large quantities in mycorrhizal roots. Such roots were heavily cutinized and the cortex and endodermis were well filled with globules of resinous or gummy appearing material. Nonmycorrhizal roots commonly contained one less row of cortical parenchyma than mycorrhizal roots. DISCUSSION The data obtained for Virginia and jack pines, and observations on uncompleted studies with white pine and Douglas fir, support in general the mineral nutrition theory of mycorrhizae as developed by Stahl (43) and Hatch (15). According to this theory (15), mycorrhizal seedlings receive better nutrition because of ( 1) the greater number of absorbing- short roots resulting from profuse branching of the mycorrhizal structure, (2) the greater surface area of the infected (mycorrhizal) short roots, (3) the delay in suberization of the cortex and endodermis, and (4) the great extension of the fungal mycelium through the soil resulting in increased surface for absorption. According to Hatch, fungi have greater capacities to acquire nutrients, especially those in organic combination, than do tree seedlings, and nutrients acquired by the fungi tend to move to the plant on a gradient. The data obtained with Virginia pine show that there were twice as many absorbing tips on mycorrhizal as on non-mycorrhizal